1. Field of the Invention
The present invention relates to a driving method for a plasma display panel (PDP) during a sustain period, and more particularly, to a driving method capable of simplifying the electrical devices required by a plasma display panel during a sustain period.
2. Background of the Invention
A plasma display panel contains an inert gas sealed within a plurality of plasma display units disposed in a matrix. A driving circuit follows a sequence causing the plasma display units to excite and ionize the dischargeable gas to emit light through its discharge. The circuit characteristics of the PDP are closely equivalent to a capacitor-like load. The driving method is to impose a high voltage and high frequency alternating current (AC) on both ends of the capacitor-like load so that the charges in the plasma display unit are driven back and forth. Fluorescent agents in the display cells will absorb the ultraviolet light radiated during the driving procedure and emit visible light.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art PDP 10. The PDP 10 comprises a back substrate 12 and a parallel, transparent front substrate 14. A plurality of sustain electrode pairs 16 are disposed under the front substrate 14. Each sustain electrode pair 16 includes sustain electrodes 18, 19 and each of the sustain electrodes 18, 19 is a bar of a constant width. A dielectric layer 20 is located under the front substrate 14 and covers the sustain electrode pairs 16. The dielectric layer 20 is utilized for providing a capacitance to prevent electric breakdown during alternating current (AC) driving. A passivation layer 22, usually made of magnesium oxide (MgO), is formed under the dielectric layer 20 for protecting the dielectric layer 20 from sputtering of plasma. A plurality of ribs 24 is located on the back substrate 12. A plurality of data electrodes 26 is disposed between the ribs 24. Blue phosphor 30B, red phosphor 30R, and green phosphor 30G are formed between the ribs 24 and above the data electrodes 26. Additionally, a discharging gas is sealed between the two adjacent ribs 24. The ribs 24 prevent the plasma on one side of the rib 24 from communicating with the plasma on the other side of the rib 24.
The sustain electrodes 18, 19 of the PDP 10 are called an X sustain electrode and a Y sustain electrode. The X sustain electrode 18 and the Y sustain electrode 19 are approximately transparent conductors with a larger width. The X and Y sustain electrodes 18, 19 are usually made of indium tin oxide (ITO), and are used to initiate and sustain a discharge. Additionally, the X and Y sustain electrodes 18, 19 comprise bus electrodes 36 and 38 respectively, located under the X and Y sustain electrodes 18, 19. The bus electrodes 36, 38 are opaque metal conductors with a narrower width. The bus electrodes 36, 38 are usually made of a chromium-copper-chromium (Cr—Cu—Cr) metal layer and are used to support the X and Y sustain electrodes 18, 19 to initiate a discharge and reduce the resistance of the X and Y sustain electrodes 18, 19.
As shown in FIG. 1, two adjacent ribs 24 and the sustain electrode pair 16 define a sub-pixel unit 32B, a sub-pixel unit 32R, or a sub-pixel unit 32G. The sub-pixel units 32B, 32R, 32G constitute a pixel unit 34. The sub-pixel units 32B, 32R, 32G and the pixel 34 are regions under the dotted lines as shown in FIG. 1. When supplying the X and Y sustain electrodes 18, 19 and the data electrodes 26 of the sub-pixel units 32B, 32R, 32G with a driving voltage, an electric field is formed to initiate a discharge of ionized gas to produce ultraviolet light, which irradiates the phosphors 30B, 30R, 30G to emit light.
Please refer to FIG. 2. FIG. 2 is a time sequence diagram of driving the PDP 10. In the PDP 10, a series of driving pulses are applied to each pixel unit through a predetermined normal driving procedure to form a set of image display pulses for displaying images. Taking the pixel unit 34 shown in FIG. 1 as an example, the normal driving procedure can be divided into a reset period, an address period, and a sustain period. When the pixel unit 34 is in the reset period, a voltage is applied to the X and Y sustain electrodes 18, 19. A main purpose of the reset period is to make statuses of wall charges on the surface of the sustain electrodes identical, which allows image data to be correctly written into predetermined addresses during the following address period. Then, the inert gas in the PDP 10 is excited and ionized to discharge, emitting light for displaying images. Because the inert gas is ionized, the pixel units of the PDP 10 are on a stable and excitable status. Prior art driving methods of the address period and the sustain period are well known to those skilled in the art so they are not described here. By repeating each period of the normal driving procedure, each pixel unit 34 of the PDP 10 receives different image display pulses and thus, users can see corresponding images displayed on the PDP 10. For example, a prior art driving method of a PDP in a sustain period is disclosed in U.S. Pat. No. 4,866,349, “Power Efficient Sustain Drivers AND ADDRESS FOR PLASMA PANEL”. In U.S. Pat. No. OLE_Link1 4,866,349 OLE_LINK1, pulses are applied on the X and Y sustain electrodes 18, 19 to excite and ionize the inert gas to discharge and emit light.
Please refer to FIG. 3. FIG. 3 is a schematic diagram of a driving circuit 40 of the PDP 10 shown in FIG. 1. The driving circuit 40 comprises capacitors C1, C2, Cp, inductors L1, L2, switches Q1, Q2, Q3, Q4, Q5, Q6, and a power supply VS, whose output voltage is V volt. The PDP 10 includes a dielectric layer 20 located between the back substrate 12 and the transparent front substrate 14, and thus producing a circuit characteristic that can be viewed as the capacitor C p formed between the sustain electrodes 18, 19. When the switch Q2 is turned on, the power supply VS inputs electrical current into the capacitor CP through the switch Q2. With the switch Q2 turned off, the power supply VS cannot input electrical current into the capacitor CP through the switch Q2. Points X, Y of the capacitor CP are connected to the sustain electrode 18 and the sustain electrode 19 respectively. The capacitors C1, C2, CP and inductors L1, L2 form a resonance circuit to make the voltages at the points X, Y of the capacitor CP oscillate. Thereby, voltages which are input into the X and Y sustain electrodes 18, 19 can be concurrently changed by the driving circuit 40 through varying the voltages of points X, Y of the capacitor CP. In addition, according to a characteristic of the resonance circuit, a voltage difference between the capacitor C1 and the capacitor C2 is equal to a half of the output voltage of the power supply (i.e. ½V volt). As the voltage difference between the capacitor C1 and the capacitor C2 is not equal to a half of the output voltage of the power supply, an energy variation will occur within the resonance circuit. The detail reasons are described as follows.
Please refer to FIG. 3 and FIG. 4. FIG. 4 is a time sequence diagram of the driving circuit 40 shown in FIG. 3 during a sustain period. Before the prior art PDP 10 enters the sustain period, all of the switches Q1, Q2, Q3, Q4, Q5, Q6 are turned off. The voltage difference between the capacitor C1 and the capacitor C2 is equal to ½V volt and the voltages of both sides of the capacitor CP are zero. Then, the switches Q1, Q5 are turned on, making the voltage of the point X oscillate from zero to V volt, wherein ½V is the voltage of the center of oscillation. That is, the amplitude of the oscillation is equal to (½V−0) volt. Turning off the switch Q and turning on the switch Q2 while the switch Q5 is still turned on, makes the voltage of the point X hold at V volt. After turning on the switch Q1, the switch Q2 is turned off while the switch Q5 is still turned on, making the voltage of the point X oscillate from V volt to zero, wherein ½V is the voltage of the center of oscillation. That is, the amplitude of the oscillation is equal to (V−½V) volt. Therefore, a pulse is produced on the point X. Then, turning off the switch Q5, the switches Q3, Q6 are turned on making the voltage of the point Y oscillate from zero to V volt, wherein ½V is the voltage of the center of oscillation. That is, the amplitude of the oscillation is equal to (½V−0) volt. Then, turning off the switch Q6, the switch Q4 is turned on while the switch Q3 is still turned on makes the voltage of the point Y hold at V volt. Next, turning off the switch Q4, the switch Q6 is turned on while the switch Q3 is still turned on makes the voltage of the point Y oscillate from V volt to zero, wherein ½V is the voltage of the center of oscillation. That is, the amplitude of the oscillation is equal to (V−½V) volt. Finally, the switches Q3, Q6 are turned off. Therefore, a pulse is produced on the point Y. If the voltage difference between the two sides of the capacitor C1 is smaller than ½V volt, the voltage of the driving circuit will be smaller than ½V volt when the switches Q1, Q5 are turned on to make the voltage of the point X rise. Therein the voltage of the driving circuit is supplied by the capacitor C1. When the switches Q1, Q5 are turned off to make the voltage of the point X drop, the voltage of the driving circuit will be larger than ½V volt. Therein the voltage of the driving circuit is supplied by the voltage difference between the power supply VS and the capacitor C1. Therefore, the energy output from the capacitor C1 is smaller than the energy input into the capacitor C1. Conversely, if the voltage difference between the two sides of the capacitor C1 is larger than ½V volt, energy output from the capacitor C1 is larger than the energy input into the capacitor C1. Accordingly, the voltage difference between the two sides of the capacitor C1 has to be equal to ½V volt in order to sustain a stable status. Similarly, the voltage difference between the two sides of the capacitor C2 has to be equal to ½V volt in order to sustain a stable status. When the prior art driving circuit 40 supplies pulses to the sustain electrodes 18, 19, it has to design resonance circuits for the sustain electrodes 18, 19 respectively to produce a pulse for each of the sustain electrodes 18, 19, wherein the pulse can oscillate from zero to V volt and then oscillate from V volt to zero. As a result, the prior art PDP 10 needs many electrical devices such as capacitors, inductors, and transistors, and thus production cost is not easily reduced.